A digital still camera equipped with an image sensor, such as a CCD, typically carries out high-sensitivity image capturing with a raised image signal level by performing a long exposure or electrically amplifying an image signal outputted from the image sensor when making an appropriate exposure under conditions of low illumination, such as at nighttime, without using any flush or the like, for example.
However, because a camera movement or a movement of the object to be photographed easily occurs during which such a digital still camera performs a long exposure, the image capturing environment is subjected to a constraint in many cases, such as a constraint that the digital still camera needs to be fixed to a tripod to shoot the object to be photographed or a constraint that any moving object, such as a human, is not chosen as the object to be photographed.
A further problem is that because the noise mixed into the image signal in the analog circuit of the camera is amplified, together with the image information, at a high amplification factor when carrying out high sensitivity image capturing, the S/N ratio of the image degrades.
To solve this problem, the following patent reference 1discloses an image processing method of using information about an image captured with a relatively low degree of sensitivity among two images captured with different degrees of sensitivity and different exposure times to remove the noise of the image captured with a relatively high degree of sensitivity.
However, because a camera movement and a movement of the object to be photographed cannot be prevented from occurring while the two images are captured even when this image processing method is used, constraints as mentioned above are still imposed on the image capturing environment.
Furthermore, although a method of using a digital pixel mixing process of adding the signal levels of neighborhood image elements within an image acquired through one image capturing without using a plurality of images to cancel random noise while raising the signal level of the object to be photographed is also used, a problem with this method is that the resolution of the object to be photographed decreases remarkably.